This invention relates generally to a method for the modification, through exposure to elemental fluorine gas, of articles molded from elastomers in order to improve the performance characteristics of the modified elastomers.
An elastomer is generally a polymeric material which possesses an inherent property known as elasticity which allows the material to return to its original form when released from a deforming load. An elastomer is capable of a substantial degree of stretching under tension before breaking. The above-referenced co-pending application, of which this application is a continuation-in-part, the disclosure of which is incorporated by reference herein, discloses a method for producing a fluorinated elastomeric article having a reduced coefficient of friction and an increased wear life. The present application discloses a method for producing an elastomeric article, and particularly a thermoplastic elastomeric article, having the above-described beneficial properties, as well as improved barrier properties, such as resistance to fluid permeability.
Thermoplastic elastomers (TPEs) have the performance characteristics of a conventional thermoset rubber with the fabrication characteristics of a conventional thermoplastic. TPEs have been categorized according to chemical composition and morphology into six commercially-available generic classes. These classes are further described in, for example, "Handbook of Thermoplastic Elastomers", 2nd Edition, B. M. Walker and C. P. Rader (Editors), Van Nostrand (1988). This reference, and all others referred to herein, are hereby incorporated by reference.
Elastomers are commonly used in a wide range of applications. These applications include, but are not limited to: static and dynamic seals, couplings, rollers, bushings, bearings, diaphragms, gears, belts, hose and tubing, springs, and shock absorbers. The performance of elastomers used in these and other applications is often limited because of the inherent friction characteristics of these materials. Several methods of improving these undesirable properties are currently in use in industry. The most common of these methods is to add to the elastomer formulation a material that is commonly referred to as an "internal lubricant". Many such materials are commercially available, including amide waxes, metallic stearates, molybdenum disulfide, various fluorocarbons, complex esters, fatty acids, polyethylenes, silicon oils, etc. When the elastomer is processed into a finished part, the internal lubricant diffuses to the surface of the article. During operation, a lubricative layer is formed between the elastomer and the mating surface, which lubricative layer lowers friction and, to some extent, increases the wear life of the elastomer. This effect, however, is often short-lived since the internal lubricant gradually wears away. Additionally, this technique introduces foreign material into the elastomer matrix and also has a tendency to induce non-uniform performance, since the lubricant's diffusion rate is controlled by temperature and pressure, both of which can vary considerably over the operational conditions.
Other techniques are also used to reduce friction in elastomeric materials. These include coating finished articles with PTFE, silicone grease or other external lubricants. Occasionally, a low friction coating may be used in tandem with an internal lubricant.
The common factor between all the techniques discussed above is that the benefits that arise from each technique are short term only. The root of the problem, i.e., the inherent high friction in certain elastomeric articles is not addressed.
It is highly desirable, therefore, to generate low friction elastomeric materials which retain their low friction and high wear resistance characteristics for substantial periods of time. It is also desirable to generate low friction thermoplastic elastomeric materials which exhibit improved barrier properties.